1. Field of the Invention
The present invention relates to directional antennae with multiple transducers, that either comprise a reduced number of the electronic channels necessary for feeding these transducers while having the same performance, in particular with respect to the level of the image lobes, or have higher performance for the same number of channels, and then includes a greater number of transducers.
The present invention is applicable in particular to sonar antennae and to sonography probes, as well for transmission as for reception. The term "feed" is used here in the broad sense as is usual for antennae, in particular microwave antennae, where it is usual to speak of a feed illuminating a reflector, even in the case of an antenna used in the reception mode. The remaining of this description will deal essentially with transmitters but the reciprocal case of the receiver will always be implied.
2. Description of the Prior Art
It is usual, as in FIG. 1, to use a linear array of transducers 10 with a width 1 and pitch d, each of these transducers being fed by a generator (or source) 20.
To generate a plane wave with a wavelength .lambda. offset by an angle .theta..sub.0 with respect to the normal to the array, the successive phase shifts .DELTA..phi. between the generators must be such that: EQU .DELTA..phi.=.phi..sub.n+1 -.phi..sub.n =(`.pi.d/.lambda.) sin .theta..sub.0.
The amplitude of the signals furnished by the generators follows a law that allows to shape the form of the radiation pattern. This directivity pattern D(.theta.) is the product of the array pattern R(.theta.) and the elemental pattern E(.theta.) of each transducer: D(.theta.)=R(.theta.).times.E(.theta.).
It is known that the pattern R(.theta.) is periodic with a period in sin .theta. equal to .lambda./d, which corresponds to phasing the waves again. Consequently, if the beam is pointed in a direction .theta..sub.0, this gives rise to image lobes in the directions .theta. such that sin .theta.= sin .theta..sub.0 .+-.k(.lambda./d) with k=1, 2, . . .
If the length 1 of the transducers is very small compared to .lambda., then E(.theta.)=1 for an .theta. and the image lobes have the same amplitude as the main lobe. The images lobes whose directions are such that -1&lt; sin .theta.&lt;1 are disturbing because they produce in the image undesired echoes that do not correspond to the direction of the formed channel and that may even mask an echo located in the pointing direction.
If these image lobes are not to be disturbing whatever the direction .theta..sub.0, it is necessary, according to the well-known rule, that d&lt;.lambda./2. If .theta..sub.0 is restricted to .theta..sub.max, we may increase d within a limit given by the relation d&lt;.lambda./(1+.vertline.sin .theta..sub.max .vertline.).
Consequently, if .theta..sub.0 is restricted to the only direction 0.degree., we have d&lt;.lambda..
In general the transducers are not punctiform and the amplitude E(Q) depends on the length 1 of the transducer compared to .lambda. according to the relation: ##EQU1##
The dimension 1 should not be to large so as not to attenuate excessively the main lobe in the directions .theta..sub.max. For example, if we admit an attenuation of -1dB for the directions .+-..theta..sub.max, we must have 1/.lambda.&lt;0.26/ sin .theta..sub.max.
For .theta..sub.max =20 .degree., the length 1 is shorter than 0.75.lambda.. As an example, FIG. 2 shows the directivity pattern obtained as a function of sin .theta. for an antenna with 18 transducers with a pitch of 1.5.lambda., each transducer having a length of 0.75.lambda., for sin .theta..sub.0 =0.18, that is .theta.=10.degree.. The curve in dashed line corresponds to the directivity pattern of an elemental transducer.
The image lobes 21 and 22 are located at -1.6 and -6.7 dB, respectively, under the main lobe 20 for sin .theta.=0.66, which is disturbing and shows that the elemental pattern in this example is not sufficiently selective.
The only solution to reduce the level of the image lobes consists in reducing the pitch between the transducers. Thus, by doubling the number of transducers to obtain an antenna with 36 transducers with the pitch of 0.75.lambda., the first image lobes will be pushed away on either side of the main lobe to a distance such that sin .theta.=1.33 . . . , i.e., twice the preceding one. The image lobes go then out of the real domain and are consequently eliminated.
The condition d&lt;.lambda./(1+.vertline.sin .theta..sub.max .vertline.) indicated above amounts to say that the phase differences at the transducers do not exceed 2.pi. between two successive transducers. These phases are called "acoustic phases".
In the prior art, the transducers are connected respectively to so many generators for transmission, or to so many reception system for reception, as there are transducers. The acoustic phases correspond then to so many electrical phases.